1. Field of the Invention
The present invention relates to setting of the thickness of a face surface of a golf club head.
2. Description of the Related Art
A low specific gravity and high strength metal material has been used for a wood club head in recent years. Size of the wood club head has been increased year by year. Therefore, the area of the face of the wood club head has been increased while the thickness thereof has been reduced. Moreover, a proposal to use flexure of the face at impact to thereby enhance carry of a hit ball has been made recently. The flexure of the face at impact has been increased as largely as possible to aim at increase in repulsion force of the face. Hence, the face has been required to have high durability without reduction in flexure at impact.
To satisfy such requirement, a proposal has been made for a face structure in which the thickness of a circumferential edge portion of the face is reduced to increase repulsion force while the thickness of a center portion of the face where hitting points are concentrated is increased stepwise by swelling back of the face toward the face center to improve durability. For example, JP-A-9-168613 has disclosed a face structure according to the proposal.
In the aforementioned proposal, however, a largest quantity of deformation, that is, maximum flexure of the face backward the head at impact occurs in a centroid of the face surface. Hence, a hill-shaped thick portion with the centroid as its peak is provided on the back of the face, or a hill-shaped thick portion with the face center, which is a point of intersection between a bisector dividing the distance from a toe to a heel into two and a bisector dividing the distance from a leading edge to a top edge into two on the face surface, as its peak is provided on the back side of the face. Therefore, a considerable reinforcing effect could be obtained if a ball was always hit on the center such as the centroid, the face center, or the like. There was however a problem that a sufficient reinforcing effect could not be obtained if a ball-hitting point on the face surface at impact was shifted from the center position (as called off-center impact).
Moreover, in the aforementioned related-art head, when off-center impact was made, the problem to be considered was that the hitting point did not coincide with the position taking the largest quantity of flexure so that sufficient repulsion force could not be obtained.
Upon such circumstances, therefore, an object of the present invention is to provide a golf club head which fulfills large repulsion force and high durability to facilitate increase in sweet area, increase in size and reduction in thickness of the head even in the case where off-center impact is made.
In order to achieve the above object, a golf club head according to a first aspect of the invention is a golf club head with a loft angle in a range of from 5 to 30 degrees, the wood club head comprising a metal body having a hollow portion and a face portion,
wherein a thick portion is formed on the back surface of the face portion, the thick portion has N steps (Nxe2x89xa72, preferably Nxe2x89xa73), when counted from behind the face portion to the face portion side, so that a center of the face portion including the first step is the thickest and the thick portion becomes gradually thin toward the peripheral portion thereof;
a centroid of a cut surface of the first step is distanced by 1 mm or more from a centroid of the face surface; and
a centroid of a cut surface of the Nth step is distanced by 1 mm or more from a centroid of a cut surface of the Nxe2x88x921th step. Incidentally, the cut surface of each of the steps is defined by cutting provisionally the face portion at each of the steps in parallel to a plane touching with the face surface at a face center;
In the golf club head according to the invention, the center portion of the face is reinforced with the thick portion provided on the back of the face. Hence, deterioration of durability of the face due to large flexure in a place near the centroid of the face surface is prevented.
Moreover, the thick portion is mounted a hill-shaped on the back of the face. Hence, reinforcement can be made in accordance with distribution of the magnitude of flexure of the face in the face surface at impact. That is, the face thickness in the center region of the face surface is set to be very large in accordance with large flexure in the center region of the face surface, and the face thickness in the peripheral region of the face surface is set to be very small in accordance with small flexure in the peripheral region of the face surface.
The thick portion is formed on the back surface of the face portion, and the thick portion has N steps (Nxe2x89xa72), when counted from behind the face portion to the face portion side, so that a center of the face portion including the first step is the thickest and the thick portion becomes gradually thin toward the peripheral portion thereof. Moreover, the centroid of the cut surface of the first step is distanced by 1 mm or more from the centroid of the face surface and the centroid of the cut surface of the Nth step is distanced by 1 mm or more from the centroid of the cut surface of the Nxe2x88x921th step. That is, the centroids of adjacent ones of the each cut surface are displaced suitably from each other. Hence, the magnitude of the face flexure taking local maximum at the centroid of the face surface as that in the related art is reduced to obtain a flat distribution leveled in a neighbor of the centroid of the face surface. Moreover, the thick portion can be formed and disposed so that the face surface is thickened most in accordance with the region in which hitting points are distributed. Hence, durability of the face surface can be improved efficiently even in the case where off-center impact in which the hitting point is displaced from the face center or from the centroid of the face surface occurs frequently. Even in the case where the head size will be increased more than that at the present time, the present invention can sufficiently cope with the increase of the head size.
Moreover, in the present invention, the position of the centroid of each of the steps constituting the face surface and the thick portion can be calculated in the condition that the face surface is frontally viewed in a direction perpendicular to a plane touching the face center while the head has a regular loft angle and a regular lie angle.
Incidentally, the xe2x80x9ccentroidxe2x80x9d or xe2x80x9cfigure centerxe2x80x9d used in the present invention is defined in known books of strength of materials and is calculated by use of the following expression in terms of distance (e) from an optional axis to the centroid, which distance (e) is a value obtained by dividing sectional primary moment (I) with respect to arbitrary axis by the total area (A).
e=I/A
Where I is a value obtained by I=∫y dA, and y is a distance from the axis to an infinitesimal area dA.
The golf club head according to the present invention is formed to have a loft angle in a range of from 5 to 30 degrees. If the loft angle is smaller than 5 degrees, it is difficult to enhance carry because even a professional golf player can hardly lift up a ball. If the loft angle is larger than 30 degrees, there is a considerable problem in practical use because the ball is typically flown too high even in the case where the head is used as a fairway club head.
Moreover, in the present invention, when respective steps constituting thick portion are provisionally cut in parallel to a plane which is a tangent touching the face surface at the face center, distance between the centroid of the cut surface of the first step and the centroid of the face surface and distance between the centroid of the cut surface of the Nxe2x88x921th step and the centroid of the cut surface of the Nth step need to be in a range of 1 mm to 25% of the face length. The both distances are preferably not less than 3 mm, more preferably 5 mm, and are preferably not more than 20% of the face length. If the distance is smaller than 1 mm, peaks of flexure distribution cannot be averaged so that region taking maximum flexure cannot be widened. This is because the maximum flexure points of respective flat plates overlap one point upon the assumption that the thick portion is formed of a laminate of the flat plates with suitable thicknesses. On the other hand, if the distance is too large, the peaks of the flexure distribution in the center region of the face cannot be averaged so that the region taking the maximum flexure cannot be widened. This is because the high bending rigidity portion of the thick portion becomes far from the centroid of the face surface so that the effect to suppress flexure in the centroid of the face surface runs short. Hence, according to the result of the inventor""s numerical analysis, it is preferable that the maximum values of the distances are not larger than 25% of the face length, preferably 20% of the face length (the horizontal distance from the toe end of the face surface to the heel end thereof). Incidentally, if the maximum values of the distances defined by only the face length are unclear, the maximum values of the distances need to be not more than 25 mm at most, preferably not more than 20 mm.
It is preferable that the thickness of each of the steps in the thick portion is selected to be in a range of from 0.1 to 1.0 mm. More preferably, the thickness of each step is in a range of from 0.2 to 0.8 mm. Further more preferably, the thickness of each step is in a range of from 0.3 to 0.6 mm. This is for the purpose of providing clear steps in the thick portion to thereby facilitate setting an arrangement of the centers of figure of the steps in production. If each of the steps is smaller than 0.1 mm, the steps are too small to set the cut surface in each of the steps. If each of the steps is larger than 1.0 mm, there is a risk that the total thickness of the thick portion becomes too large.
In relation to a total thickness from the face surface to top surface of each step, preferably, the maximum of the total thickness (the thickest position) is not more than 5.0 mm, and the minimum thereof (the thinnest position) is not less than 1.0 mm. More preferably, the maximum of the total thickness (the thickest position) is not more than 3.5 mm, and the minimum thereof (the thinnest position) is not less than 1.5 mm. If the maximum thereof is more than 5.0 mm, an effect of the reinforcement is not only increased, but it also goes against a requirement of weight saving. If the minimum thereof is less than 1.0 mm, an effect of the thick portion is not obtained sufficiently. Since the number of the steps are limited by the thickness of the thick portion and the thickness of each step, the maximum of the number of steps are not limited specifically.